Free field stress gauges defined herein are gauges used to measure stresses in the medium in which they are used by physically embedding them in the sensed medium, e.g. a geological rock and/or soil formation, and in civil engineered structures such as roads or buildings. Specific applications of this sensor include use in passive smart structures to assess the state of a civil engineered structure in real time such as a skyscraper's pilings in an an earth quake prone area. Additional uses include placement in a road for traffic light control.
The current invention is an improvement over two known earlier free field stress gauges.
The gauge taught by Richard Kanda's U.S. Pat. No. 4,092,856 entitled "Kilobar Range Stress Gauge," which is hereby incorporated by reference, teaches of a concentric hollow ring central sensing column in a first embodiment and a central sensing column in a second embodiment. Another gauge by U.S. Army Corps of Engineers Waterway Experiment Station Technical Instruction Report SL-86-1, dated September 1986, entitled "Procedure for Assembling WES Column-Based Soil Stress Gages" (referred to as COE 86 gauge) teaches of a soil stress gauge that is capable of both static and dynamic stress measurements up to 50 ksi in various types of soil.
Kanda's '856 gauge teaches of a stress gauge designed to match the density and Young's modulus of the surrounding medium and operate up to one kilobar stress ranges. The instant invention's gauge material approximately matches the density of the sensed medium, and is also intentionally designed to have an average elastic Young's modulus in the direction of its measuring axis that is higher than the surrounding sensed medium resulting in a higher ratio of support area to loaded area for a preferred axial orientation of the gauge. This in turn results in a gauge with much higher measuring ranges in comparison with the Kanda's '856 gauge. For example, the present invention can be fabricated from 4340 alloy steel, that is heat treated to a yield strength of 199,000 psi and operate in ranges of approximately 145,000 psi (10 kilobars).
Another limitations of the Kanda's '856 gauge is it's required matching of the Young's modulus of the sensed medium with that of the gauge which results in limited applicability to sensed medium that are linear elastic, i.e. sensed mediums where Young's modulus of elasticity is also constant. The instant invention's gauge differs in that it uses the fact that inclusions of relatively high Young's modulus in sensed mediums of relatively low Young's modulus over-register the free field stress by almost constant ratio. Thus, if the surrounding sensed medium's Young's modulus varies during loading or unloading, then the instant invention's gauge will provide continuous accurate monitoring of the free field stress via an "over-registration" or calibration factor, with only the constraint that the sensed medium's Young's modulus remain less than the present invention's gauge.
Yet another inherent limitation of Kanda's '856 gauge design is its structure having a Young's modulus equal to the sensed medium which in turn limits its useful applications. The instant invention's gauge design requires only a single gauge design for sensing stress in a great number of diverse media without regard for a uniformity of the gauges Young's modulus with that of the sensed medium.
Another limitations of the Kanda's '856 gauge includes the type of unspecified stress gauges used in the Wheatstone bridge electrical circuit which affects the gauge's longitudinal (axial) and circumferential strain sensitivity. Kanda does not discriminate as to the type of strain gauge that should be used. The present invention uses a combination of semi-conductor strain gauges for measuring longitudinal strains and foil type strain gauges for measuring circumferential strains on the sensing column for greater gauge sensitivity to normal stress, and reduced sensitivity to lateral stress. Additionally, the COE 86 gauge uses strictly semi-conductor gauges which is a disadvantage since the improved longitudinal (axial) sensitivity to circumferential sensitivity is not achieved.
Still another limitation of the Kanda's '856 and the COE 86 gauge are that the actual sensing column is entirely within the cover disc which requires these devices have very tight machining tolerances in order for the active disc and cover disc to cooperate properly for proper gauge operation. Additionally, the sealing means between these disc is not as effective as the present invention's when under intense dynamic loading where harmful fluids can ingress into the central gauge areas where electrical connections and strain gauges are located and cause failure. The present invention obviates these problems by having a portion of the sensing column protrude through the gauge cover and makes direct contact with the ambient sensed medium. Moreover, the invention herein includes a positive double watertight sealing means between the top cover disc and the bottom active disc whose sealing quality is enhanced when the gauge is loaded. Thus, the invention's design obviates the need for high machine tolerances in its manufacture and allows for greater gauge reliability compared to previous gauges.
Yet still another limitation of the Kanda's '856 gauge and the COE 86 gauge is their increased hysteretic effects due to the sensing column of both these gauges being entirely within the cover disc and loading on the column is indirectly effected by normal stresses of the surrounding medium acting on the cover disc where reseating of the gauge parts occurs after the gauge is loaded. In contrast, the present invention obviates this effect by having most of the top loaded area of the sensing column directly exposed to the sensed medium. The portions of the sensing column's top loading area which are not in direct contact with the sensed medium, are also connected to the sensed medium via relatively flexible members.
Yet another limitation of the Kanda's '856 gauge and the COE 86 gauge is caused by their respective gauge geometries which limits the elastic range of use. Kanda's '856 gauge is limited to fields of around one kilobar, i.e. 15,000 psi range for aluminum alloy 7075-T6. The present invention's gauge design allows for gauge elastic range to be quite high, of around 70 percent or more of the elastic range (in compression) Of the material used for gauge construction, which translates into operating capabilities of up to 10 kilobar range of measurements as discussed above.
Still another limitation of the Kanda's '856 gauge and the COE 86 gauge is the way signal wires are routed through and supported by their gauge's active disc structure. With acceleration induced flexure that occurs during explosive tests, gauge failure often occurs due to severed sensing wires or interconnection failures. The present invention resolves such problems by providing tight and well-supported sensing wires and filling voids inside the gauge body with a light-weight stabilizing strong material that further supports the internal signal wires, strain gauges, solder joints and tabs from inertial loading.
Yet still another limitation of the COE 86 gauge is its large size which affects time and the frequency response capabilities during transient operation. One aspect of the present invention has proportionally reduced diameter and thickness by approximately 20 percent compared to the COE 86 gauge while still having the desired diameter-to-thickness ratio of five or better as required for free field stress gauges, see the article by Peattie et al. entitled "The Fundamental Action of Earth Pressure Cells," in Journal of the Mechanics and Physics of Solids, 1954, Vol. 2, pp. 141-55. The present invention reduces required gauge size which allows the gauge to be used in small areas, reduces the volume over which the stress field is perturbed by the presence of the gauge itself, and provides a gauge which has a higher frequency response.